1. Field of Invention
The present invention relates generally to apparatus and method to encode and decode position information on a recording medium and more particularly, but not by way of limitation to improvements in encoding and decoding position information on a recording medium for use with rotating disc magnetic data storage devices.
2. Brief Description of the Prior Art
In rotating disc magnetic data storage devices, data is stored in sectors extending angularly along concentric data tracks defined on the discs of the device. The discs have magnetizable surface coatings. Data is written and subsequently read by transducer heads that fly over the surfaces of the disks to magnetize cells of the surface coatings, for writing, or respond to differences in magnetization of adjacent cells for reading. Both operations are controlled by a read/write controller that provides encoded data to the transducer head during writing and receives pulses from the transducer heads during readback of the data.
Further, the data is written on the discs either at constant frequency, with a single zone of recorded data or at variable frequency with multiple zones of recorded data. Each zone will have multiple subtracks. The subtracks further have multiple sectors. Thus, in order to read data from a sector or write data to a sector, there is a need to locate and identify a sector. Thus, there is a need to determine various position determining information like the subtrack number, zone number and the sector number. Typically, this is accomplished by first writing a position information data field in every sector and subsequently reading the information during a positioning operation. The position information data field typically contains a Fill field, an Automatic Gain field, an Index field. a Position Identifier field and a Servo Burst field. The Position Identifier field contains a position identifier, typically a global subtrack number encoded in Gray Code. The subtrack number is a unique number identifying a specific subtrack in a specific zone. The index field contains an index mark bit. The index mark bit in the first sector will be different than the index mark bit in all other sectors. Thus, the orientation of the transducer with respect to the disc surface includes first establishing the angular position then locking an oscillator with respect to the disc. During a positioning operation, when the transducer encounters the index mark of the first sector, it enables a sector counter which indicates when the next index mark should be encountered based upon the speed of the disc rotation. When the transducer encounters the second index mark it is validated against the indication of the sector counter. If the actual index mark read by the transducer coincides with the indication of the sector counter, it is concluded that the sector counter is providing accurate index synchronization. The sector counter is further used to count the sector numbers. Position Identifier contained in the position identifier field provides the subtrack number of the subtrack within the zone.
FIG. 1 shows position identifiers written on a disc media having four zones 1 through 4, with each zone having five subtracks 0 through 4 and each subtrack further having four sectors, according to a prior art system. A unique position identifier 21 is written in each subtrack, so that a specific subtrack in a specific zone can be identified. A sector counter is maintained in the control system to identify each sector in a subtrack. As it can be seen, using the prior art system, there is a need for twenty (number of zones x number of subtracks) unique position identifiers to identify all the subtracks and the zones. Further, the control system needs to maintain the sector count, so that the specific sector on which the head is positioned is determined.
In the prior art systems as described above, there is a need to write a unique index mark bit in the position data field of the first sector. Further, there is a need to write an index mark in every sector. In addition, there is a need to write a unique Position Identifier in Gray Code for every subtrack contained in all the zones. The need to write a physical index mark bit in every first sector can pose problems during the manufacture of the disc drive if the location where the index mark has to be written is defective. In addition, the index mark bit in the first sector may deteriorate or be accidentally overwritten. As the number of tracks and subtracks in a disc drive increase, the number of bits needed to encode a unique position identifier in gray code for every subtrack will increase.
There is a need to eliminate or provide an alternate index mark bit to locate or validate the start of a subtrack and the sector count. There is also a need to provide an improved method of encoding the subtrack number and the zone number so that less number of bits are needed to encode the position identifiers. There is also a need to provide an encoding method whereby redundant information is available for a controller to accurately determine the position of the transducer head viz. sector number, subtrack number and zone number.